DNV and Tension Tech Study Standards for Mooring Lines

Fiber ropes are used as mooring lines on deepwater drilling and production platforms, but there has been a need for better design data. DNV and Tension Technology International have now conducted a joint industry research project to develop a new testing standard which will further facilitate the use of such ropes through more secure design and enhanced cost effectiveness.

The joint industry project was sponsored by leading oil companies and contractors, with broad support from the rope-making industry.

"There is a great need for a simple standardized test method to determine the fiber-rope visco-elastic change-in-length characteristics," explains Vidar Ahjem, DNV's representative on the project. 

"Accurate data on rope stiffness and stretch are important when designing deepwater mooring systems. Stiffness affects the required rope break strength, and permanent stretch affects the required rope length. Further, stiffness and stretch influence the maximum platform offset and thus required riser length," elaborates John Flory, who represents TTI in the project.

Designers of polyester mooring systems generally use the results of only a few special tests on how the rope will stretch during installation and storms. Such tests are expensive and time consuming, and are applicable to only one project. The newly developed standardized test method will more efficiently determine the change-in-length performance of large polyester ropes, independent of the loading scenario. So now the test needs to be run only once for a particular rope design, and the test results can then be used on any project.

Use of the Data
The mooring system designer can now use the test results to predict the change-in-length properties of the rope after any loading history and in response to any change in load. It will no longer be necessary to carry out the customary series of tests for each deepwater mooring system design.

The required mooring line length and take-up can be calculated based on an assumed installation procedure. If the installation procedure changes, for example as a result of installation vessel availability, the calculation can be rerun without having to retest the rope.

Peak mooring tensions and maximum mooring line stretches in response to hurricanes and loop currents can readily be calculated. The total permanent stretch over the service life of the mooring can be accurately predicted, and the required length take-up can be determined.

This 'standardized' test method  provides data which are applicable for any installation or service situation
Accurate rope change-in-length data are available at the beginning of the design process, as well as throughout the phases of a project, from concept study to operation.

The fiber rope can be optimized by selecting the proper strength, stiffness, and length of rope.

Implementation of the New Test Method
The new test method will be released in the next edition of the DNV Offshore Standard DNV-OS-E303 "Certification of Fiber Ropes for Offshore Mooring". Before that time, an updated version will be issued internally to the JIP sponsors so that they can benefit from this new method immediately.

DNV is a global provider of services for managing risk, helping customers to safely and responsibly improve their business performance. DNV is an independent foundation with the purpose of safeguarding life, property and the environment. Through its network of 300 offices in 100 countries, the company serves a range of industries, with a special focus on the maritime and energy sectors, combining its technology expertise and industry knowledge.

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